The present invention relates generally to a diffractive optical element (hereinafter DOE for short) comprising a diffractive surface having lens action based on diffraction phenomena, and more particularly to a lens system comprising a single lens both surfaces of which are constructed of diffractive surfaces.
In optical systems used on silver salt cameras, electronic cameras or the like, much more lenses and much more sophisticated arrangements are required to satisfy much higher performance, as represented by phototaking lenses. However, all available optical systems are not always structurally complicated; some optical systems are made up of one single lens. One example is an active range finder as shown in FIG. 1. This is based on the principle of trigonometric measurement as explained briefly below with reference to FIG. 1. Reference numeral 11 is an infrared-emitting diode or IRED; 12 a light projecting lens element for projecting infrared light emitted from IRED 11; 13 a subject; 14 a light receiving element for receiving light reflected from the subject 13; and 15 a position sensing device or PSD for sensing the position of the received light. IRED 11 emits infrared light, which is in turn projected through the projecting lens element 12 on the subject 13. Light reflected by the subject 13 is focused on PSD 15 through the receiving lens element 14 positioned away from the projecting lens element 12 by a base length. The subject distance is calculated from position information on PSD 15.
Light-projecting, and -receiving lens elements used on such an active range finder are often made up of one single lens. Though some lens elements are produced in the form of prisms having a reflecting surface, yet they are fundamentally composed of one single lens. These light-projecting, and -receiving lens elements are unavoidably increased in both diameter and thickness because brightness is of importance.
A lens element used on photometric devices for external photometry is again made up of a single lens. This is mounted within a camera body separately from other parts such as a phototaking lens and a finder lens to make a photometric measurement of the subject, as explained below with reference to FIG. 2. Reference numeral 21 is a condenser lens; 22 a filter; and 23 a light-receiving element. The filter 22 is to bring the spectral sensitivity of the light-receiving element 23 in conformity to film properties, and has fundamental action on cutting infrared light. The condenser lens 21 is often made up of one single lens for the purpose of achieving compactness and cost reductions. However, this lens is again unavoidably increased in size because sufficient brightness and the angle of photometry in conformity to the field angle of a photo-taking lens are needed.
The phototaking lens, too, is made up of a plastic single lens when it is used on inexpensive cameras as represented by a combined lens and film camera which, as schematically shown in FIG. 3, comprises a phototaking lens 31 made up of a single lens, an aperture stop 32, and a film surface 33. The film surface 33 is bent along its longitudinal direction and concave on the subject side. A single lens has some degrees of freedom in reducing spherical aberration or low order coma, but has no room for a choice of bending shape. In other words, the single lens is often designed in the form of a meniscus lens having a concave surface directed toward a stop, and so there is no room for making its overall length short because lens shape is predetermined in view of correction of aberrations.
For recently developed cameras, on the other hand, considerable size reductions have been desired. Many parts are mounted in a camera body. To achieve compactness, it is required to reduce the number and size of these parts. For such various lens elements as mentioned above, too, it is required to reduce their size. In view of such situations, an object of the present invention is to make a single lens smaller, especially thinner than ever before, by making use of a diffractive optical element.
Here, the diffractive optical element or DOE is explained. For details of DOE, however, see "Optics", Vol. 22, pp. 635-642, and 730-737.
While a conventional lens is based on the refraction of light at a medium interface, DOE is based on the diffraction of light. Now consider the incidence of light on such a diffraction grating as shown generally in FIG. 4. Emergent light upon diffracted satisfies the following equation (a): EQU sin .theta.-sin .theta.'=m.lambda./d (a)
where .theta. is the angle of incidence, .theta.' is the exit angle, .lambda. is the wavelength of light, d is the pitch of the diffraction grating, and m is the order of diffraction.
Therefore, if the pitch of a ring form of diffraction grating is properly determined according to equation (a), it is then possible to converge light on one point, i.e., impart lens action to the diffraction grating. Here let r.sub.j and f the radius of a j-th ring on the grating and the focal length of the diffractive surface, respectively. Then, the following equation (b) is satisfied in a region of a first approximation: EQU r.sub.j.sup.2 =2j.lambda.f (b)
For a diffraction grating, on the other hand, a bright-and-dark ring form of amplitude-modulated type grating, and a phase-modulated type grating with a variable refractive index or optical path length has been proposed. In an amplitude-modulated type DOE, for instance, the ratio between the quantity of incident light and the quantity of the first order of diffracted light is about 6% at most because plural orders of diffracted light are produced. Hereinafter, this ratio will be called the "diffraction efficiency". Even though this amplitude-modulated type DOE is modified as by bleaching into the phase-modulated type, the diffraction efficiency is about 34% at most. If the same phase-modulated type DOE as mentioned above is modified such that its section is of such saw-toothed shape as depicted in FIG. 5(a), however, the diffraction efficiency can then be increased to 100%. Such a DOE is called a kinoform. In this case, the height of each tooth is given by EQU h=m.lambda./(n-1) (c)
where h is the height of the tooth, and n is the index of refraction of material.
As can be predicted from equation (c), a diffraction efficiency of 100% is achievable at only one wavelength. The kinoform shape may be stepwise approximated as shown in FIG. 5(c) to obtain a so-called binary optical element. This element can be relatively easily fabricated by lithography techniques. As well known in the art, the binary optical element has a diffraction efficiency of 81% to a four-step approximation, 95% to an eight-step approximation, and 99% to a sixteen-step approximation.
DOEs may be designed by some known methods. However, the present invention makes use of an ultra-high index method as set forth in an article "Mathematical equivalence between a holographic optical element and ultra-high index lens", J. Opt. Sos. Am. 69, pp. 485-487 or an article "Using a conventional optical design program to design holographic optical elements", Opt. Eng. 19, pp. 649-653. That is, the DOE is known to be equivalent to a refractive surface having null thickness and an ultra-high refractive index.
A DOE has two important features when used in the form of a lens. The first feature is aspheric action. If the pitch of a diffraction grating is properly determined, it is then possible to converge light perfectly on one point. This is tantamount to reducing spherical aberration to zero by use of an aspheric surface. The second feature is that chromatic dispersion is very large or, in another parlance, an Abbe number of -3.45 is obtainable. Chromatic aberration several tens times as large as that of a conventional refractive material is produced with a minus sign or in the opposite direction. Large dispersion offers the gravest problem when the ODE is applied to a lens element used under natural light. The refractive index of DOE at any wavelength is given by EQU n(.lambda.)=1+[n(.lambda..sub.0)-1].multidot..lambda./.lambda..sub.0(d)
where k is any wavelength, n(.lambda.) is the refractive index of DOE at wavelength .lambda., .lambda..sub.0 is a reference wavelength, and n(.lambda..sub.0) is the refractive index of DOE at wavelength .lambda..sub.0.
An example of applying such a DOE to an active range finder is disclosed in JP-A 7-63982. This publication shows that zooming is carried out with a converter lens inserted on the IRED side of a master lens, and that the master lens is in a plano-convex form while the converter lens is in a plano-concave form, with each plane made up of a diffractive surface. Thus, zooming is achievable while the master lens remains fixed. However, this publication says nothing about how compactness is achieved.
An example of applying a DOE to a phototaking lens is set forth in an article "Hybrid diffractive-lenses and achromats", Appl. Opt. 27, pp. 2960-2971. This prior publication shows an example of calculation in the case where, based on the principle of correction of paraxial chromatic aberration, a diffractive lens having an Abbe number of -3.45 is used in combination with a conventional refractive lens to make correction for chromatic aberration. Specifically, the publication shows a lens with the object-side surface constructed of a convex surface and the image-side surface constructed of a plane surface, wherein a diffractive surface is formed on the image-side plane, and refers to the achromatization of axial chromatic aberration and the remaining secondary spectrum. However, this publication does neither refer to chromatic aberration of magnification and other aberrations nor give any specific design data.
WO95/18393 shows an arrangement wherein a positive meniscus lens convex on a subject side and a stop are positioned, and an image-side surface of the positive lens is constructed of a diffractive surface. This publication teaches that chromatic aberration is corrected by a combined refractive and diffractive system, and alleges that high performance is achieved without any increase in the number of lens parts.
Both publications directed to the application of DOEs to phototaking lenses are primarily to make correction for chromatic aberration and state that compactness is achievable by reason of any increase in the number of lens parts. However, they fail to provide a disclosure of how the overall length of a single lens is reduced.